This invention relates to a filling fitting for filling a closed hot-water heating system, comprising: an inlet and an outlet to the heating system, a water path being defined between said inlet and said outlet, a tube separator including an upstream backflow preventer, a downstream backflow preventer and a drain valve controlled by the inlet pressure and arranged, with regard to the flow, between said two backflow preventers, and a pressure reducer.
Hot-water heating systems occasionally have to be refilled by water from the fresh water supply system. When refilling the system, it has to be ensured, that no heating water from the hot-water heating system flows into the fresh water supply system thereby contaminating the fresh water. Thus, it is not possible just to connect the hot-water heating system to a fresh water supply pipe through a stationary connection pipe having a shut-off valve. This could lead to the fact, that heating water flows back from the hot-water heating system into the fresh water supply system if, for example, the shut-off valve is open and the pressure in the fresh water supply system for some reason drops strongly. In order to prevent this, "backflow preventers" are provided in the water path between the hot-water heating system and the fresh water supply system. Backflow preventers are spring-loaded check valves adapted to open in the filling direction. When the system is filled these backflow preventers open under the action of the inlet pressure of the fresh water supply system and allow water to flow into the hot-water heating system. However, the backflow preventers close when the inlet pressure drops below the pressure in the hot-water heating system. Thus, they prevent backflow of heating water into the fresh water supply system.
Such backflow preventers may occasionally leak. That is why in some countries a stationary connection between fresh water supply system and hot-water heating system is not allowed even if a backflow preventer is provided. The connection has to be removed, after the system has been filled. This can be done in that filling or refilling of the hot-water heating system can be effected only through a hose which is removed after the filling. This is cumbersome. Furthermore, it is not ensured, that the hose actually is removed after the filling and the connection to the fresh water supply system has actually been removed. For this reason, "tube separators" are used. Such tube separators include an upstream backflow preventer, a downstream backflow preventer and a drain valve controlled by the inlet pressure and arranged, with regard to the flow, between the two backflow preventers. In such tube separators the drain valve usually consists of a tube connecting element located in the water path between the inlet and the outlet to the system, the tube connecting element being movable against a restoring force by an actuator piston exposed to the inlet pressure from an open position, in which the water path is interrupted and open toward a drain, to a closed position, in which the tube connecting element closes the water path. A shut-off valve is connected upstream from the tube separator. The tube separator and the shut-off valve are permanently installed. When the shut-off valve is closed in the normal state, the inlet pressure does not act on the actuator piston. The tube connecting element is retracted toward the inlet side closing the water path. Thus, it is ensured, that heating water cannot flow into the fresh water supply system if, for example, the shut-off valve unintentionally is kept open or the backflow preventer leaks. The downstream backflow preventer prevents the hot-water heating system from being drained. Heating water flowing out of the hot-water heating system when the downstream backflow preventer leaks flows into the drain. When the shut-off valve is opened in order to refill the hot-water heating system, the inlet pressure acts on the actuator piston. Thus, the tube connecting element is advanced and closes the water path to the hot-water heating system. During this procedure the upstream backflow preventer still is in the closed position. The upstream backflow preventer does not open until a further pressure rise occurs, after the tube connecting element has engaged a stop and has closed the water path.
In order to prevent inadmissibly high pressure in the hot-water heating system, which could be caused by high pressure in the fresh water supply system, a pressure reducer is inserted in prior art filling fittings between the connection to the fresh water supply system and the hot-water heating system. Such a pressure reducer comprises a control valve in the water path. The control valve is controlled by a diaphragm, on which the system pressure acts against the action of a spring. Normally, provision is made to temporarily disable the control function of the pressure reducer, when the hot-water heating system is being filled. In order to allow maintenance of the pressure reducer, usually a further shut-off valve is connected downstream from the pressure reducer, by means of which the connection between pressure reducer and hot-water heating system can be shut off by hand.